1. Field of the Invention
The present invention relates to surface acoustic wave devices and, more particularly, to a surface acoustic wave device which improves the balance within the passband thereof, particularly an amplitude balance thereof, and to a communication apparatus incorporating the surface acoustic wave device.
2. Description of the Related Art
Rapid advances in the field of mobile telephones have achieved compact and light-weight designs. Along with technical advances, a plurality of functions are incorporated in a single component in accordance with efforts to reduce the number of components of the mobile telephone and to miniaturize the mobile telephone.
Surface acoustic wave filter devices for use in an RF stage in a mobile telephone have been actively studied to incorporate a balanced-to-unbalanced line-coupling function, or a so-called balun function. Such filter devices have been used, particularly, in GSM (Global System for Mobile Communications). Furthermore, such filter devices are likely to be used in PCS (Personal Communication Services) and DCS (Digital Communication System).
Japanese Unexamined Patent Application Publication 6-204781 (with publication date Jul. 22, 1994) and Japanese Unexamined Patent Application Publication 11-145772 (with publication date May 28, 1999) disclose surface acoustic wave filter devices having the balanced-to-unbalanced line-coupling function.
FIG. 17 is a plan view schematically illustrating a surface acoustic wave filter device having a balanced-to-unbalanced line-coupling function.
Referring to FIG. 17, a surface acoustic wave filter device 1R includes a first surface acoustic wave filter 100 provided on a piezoelectric substrate (not shown), and a second surface acoustic wave filter 200 having a phase different by 180° from a phase of the first surface acoustic wave filter 100.
The first surface acoustic wave filter 100 is a serially coupled three-IDT (interdigital transducer) resonator type surface acoustic wave filter including a center IDT 101, two IDTs 102 and 103 on both sides of the center IDT 101, and reflectors 104 and 105 with the two IDTs 102 and 103 interposed therebetween. The second surface acoustic wave filter 200 is different in phase by 180° from the first surface acoustic wave filter 100 with the polarity of electrodes 206 and 207 set to be opposite to the polarity of electrodes 106 and 107 in the first surface acoustic wave filter 100. Terminals 52 and 52 are balanced signal terminals, and a terminal 51 with which terminals 112 and 212 are electrically connected in parallel is an unbalanced signal terminal 51.
The filter having the balanced-to-unbalanced line-coupling function must satisfy a requirement that transmission characteristics within a passband of the filter from the unbalanced signal terminal to the two balanced signal terminals remain equal in amplitude but different in phase by 180°, and a requirement that transmission characteristics out of the passband of the filter from the unbalanced signal terminal to the two balanced signal terminals remain equal in amplitude and in phase.
An amplitude balance and a phase balance are defined as below. A surface acoustic wave filter device having the balanced-to-unbalanced line-coupling function is now considered as a three-port device with the port 1 defining as an unbalanced signal input terminal, and the ports 2 and 3 respectively defining the balanced signal output terminals.
 Amplitude balance=|A|A=|20 log S21|−|20 log S31|  Equation 1Phase balance=|B−180|B=|<S21−<S31|  Equation 2where S21 and S31 are elements of a scattering matrix into which the three-port device is expressed, and respectively represent transmission characteristic between the port 2 and the port 1, and the transmission characteristic between the port 3 and the port 1.
Ideally, the amplitude balance is 0 dB and the phase balance is 0 degree within the passband of the filter, and the amplitude balance is 0 dB and the phase balance is 180 degrees out of the passband of the filter.
A deviation from the ideal balance occurs in the conventional surface acoustic wave filter device 1R shown in FIG. 17, and the degree of deviation becomes a problem in the actual operation of the filter.
In the first surface acoustic wave filter 100, electrodes 107, 108, and 110 having electrode fingers at each of borders between the three IDTs 101, 102, and 103 are grounded. In the second surface acoustic wave filter 200, an electrode 206 having electrode fingers at each of borders with the IDTs 201, 202 and 203 is connected to the signal terminal 51, and electrodes 208 and 210 having electrode fingers at each of borders with the IDT 210 are grounded.
Since grounded electrode fingers are arranged side by side at the borders of the IDTs in the first surface acoustic wave filter 100, no surface acoustic wave is excited in this portion. On the other hand, since the electrode fingers connected to the signal terminal and the grounded electrode fingers are arranged side by side at the borders of the IDTs in the second surface acoustic wave filter 200, a surface acoustic wave is excited.
The number of electrode fingers in the IDT 101 in the first surface acoustic wave filter 100 is different from the number of electrode fingers in the IDT 201 in the second surface acoustic wave filter 200. The first surface acoustic wave filter 100 and second surface acoustic wave filter 200 have substantially different filter characteristics from each other.
The difference in the filter characteristics is a major cause of significant degradation of the balance, particularly within the passband.
Japanese Unexamined Patent Application Publication No. 59-107620 (with publication date Jun. 21, 1984) discloses a technique in which an impedance element including a resistor, a coil, and a capacitor is connected to a balanced signal terminal to improve balance.
In practice, it is difficult to include a desired impedance element within a surface acoustic wave filter device. If such an impedance element is attached as an external element, an extra mounting area is required and costs are increased accordingly.